1. Field of the Invention
The present invention relates to a superconducting thin film and a process for depositing the same on a substrate.
More particularly, it relates to a thallium (TI) type superconducting thin film possessing an improved high critical current density (Jc) as well as a high critical temperature (Tc) and a process for depositing the same on a substrate.
2. Description of the Related Art
The superconductivity is a phenomenon which is explained to be a kind of phase change of electrons under which the electric resistance become zero and the perfect diamagnetism is observed.
Several superconducting devices have been proposed and developed in electronics which is a typical field to which the superconducting phenomenon is applicable. A typical application of the superconductor is Josephson device in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. The tunnel junction type Josephson device is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high-sensitive sensor or detector for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a precise quantum phenomenon. Development of the superconducting devices such as high-speed logic units or of no power-loss wiring materials is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption. The critical temperature "Tc" of superconductivity, however, could not exceed 23.2K of Nb.sub.3 Ge which was the the highest Tc for the past ten years.
The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64, 1986 p189).
It had been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi-Pb type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10K and hence usage of liquidized helium (boiling point of 4.2K) as cryogen is indispensable to realize superconductivity.
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher Tc as 30K which are extremely higher than known superconducting materials.
It was also reported that C. W. Chu et al. discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90K in February 1987 (Physical Review letters, Vol. 58, No. 9, p908).
The other type new superconducting materials which were reported recently are a compound oxide of Bi-Sr-Ca-Cu-O system reported by Maeda et al (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210) and Tl-Ba-Ca-Cu-O system which exhibit such high Tc as more than 100K (Hermann et al. Appl. Phys. Lett. 52 (20) p1738) and which are chemically much stable than the abovementioned YBCO type compound oxide or the like. And hence, the possibility of an actual utilization of the high Tc superconductors have burst onto the scene.
The above-mentioned new types superconducting materials were prepared in a bulk form of sintered block which was obtained by sintering a powder mixture of oxides or carbonates of constituent metal elements. They can be deposited on a substrate in a form of a thin film by physical vapour deposition (PVD) technique or chemical vapor deposition (CVD) technique. In both cases, it is a usual practice to subject the resulting sintered blocks or thin films to heat-treated in an oxygen-containing atmosphere to adjust the oxygen deficiency in the crystal.
The present applicant already proposed several processes for preparing the thin films of the high-Tc superconductor on a substrate in the following commonly assigned patent applications: U.S. patent application Ser. No. 152,714 filed on Feb. 2, 1988, now U.S. patent application Ser. No. 167,895 filed on Mar. 13, 1988, now abandoned, U.S. patent application Ser. No. 195,145 filed on May 18, 1988, now U.S. Pat. No. 4,900,716, U.S. patent application Ser. No. 195,147 filed on May 18 1988, now abandoned U.S. patent application Ser. No. 200,206 filed on May 31, 1988, now U.S. Pat. No. 4,996,185, U.S. patent application Ser. No. 286,860 filed on Dec. 20, 1988, now U.S. Pat. No. 5,028,583, U.S. patent application Ser. No. 289,718 filed on Dec. 25, 1988, now abandoned U.S. patent application Ser. No. 289,719 filed on Dec. 25, 1988, and U.S. patent application Ser. No. 290,309 filed on Dec. 26, 1988 now abandoned, or the like. The present invention completed on the same line as these patent applications.
The present invention concerns the superconducting thin films of thallium (Tl) type compound oxide. This type superconducting thin film has been prepared by deposition techniques such as RF sputtering, vacuum deposition or MO-CVD technique. They posses, however, very low critical current density (Jc) although they showed very high critical temperature (Tc), so that they were difficult to be utilized in practical uses.
An object of the present invention is to overcome the conventional thallium (Tl) type superconducting thin films and to improve the critical current density (Jc) and a process for preparing the same.